Midrasch zu Bamidbar 9:22

And the Lord went before them by day (Exod. 13:21). You find that there were seven clouds of glory in all. And the Lord went before them by day in a pillar of cloud (ibid.) was the first; and Thy cloud standeth over them (Num. 14:14) was the second; In a pillar of cloud (ibid.) was the third; That the cloud tarried (Num. 9:22), the fourth; And the going up of the cloud (Exod. 40:36), the fifth; And whenever the cloud was taken up (ibid., 37), the sixth; and For the cloud of the Lord was upon the Tabernacle (ibid., v. 38) was the seventh. Four of the clouds were at each of the four sides; one was above them, another below them, and the seventh cloud preceded them. The last of the clouds lowered the high places, raised the low places, burnt the serpents and the scorpions, and sprinkled the road (to lay the dust) before them.